- Detail

Finite element calculation of circuit parameters of large hydro generator

1 background of the problem

the rapid development of the power industry and the fair and correct use of the fixture of the generator are conducive to the smooth progress of the experiment, the continuous increase of the capacity of a single machine, which puts forward higher and higher requirements for the safe operation and reliability of the generator, early detection of accident hidden dangers, and improvement of fault diagnosis and protection level, It has become an important research topic in the electrical field [1,2]

when faults such as inter turn short circuit, inter phase short circuit and open welding break occur in the stator winding of the generator, the three-phase winding of the motor is in an asymmetric operation state, the spatial harmonic component of the air gap magnetic field is very strong, and there are many harmonics of the winding induced emf, which is the prominent feature of AC motor when the winding is asymmetric. For the problem of motor performance changes caused by the asymmetry of motor windings, the theories and methods commonly used in the past, mainly based on the motor as the analysis object and the symmetrical phase winding, can no longer meet the requirements. Instead, the motor and its associated external devices need to be taken as a system to consider the influence of electromagnetic harmonic components. For this reason, document [1] puts forward the multi loop theory of the motor, which discusses the basic electromagnetic relationship of the motor based on a single coil element, and establishes the corresponding relationship, so as to go deep into the interior of the winding and reasonably analyze the internal faults of the winding [3]

the key of the multi loop analysis method is the calculation of the loop parameters after the fault. With accurate loop parameters, the electrical quantities after the internal short-circuit fault of the stator winding can be calculated more accurately in theory. The constant consistency of these loop parameters refers to the self inductance and mutual inductance of each loop when the motor is regarded as multiple loops coupled with each other due to the influence of wind. Because the air gap magnetic conductivity method can consider the effects of air gap magnetic field harmonics and current harmonics, and the amount of calculation is small, it is a fast and effective method to obtain generator winding circuit parameters [4]. However, the air gap permeance method reduces the core reluctance to the air gap, and does not consider the nonlinearity of the core reluctance, the air gap magnetic potential and the tooth harmonic of the air gap permeance. Using these loop parameters to solve the electric quantity value under fault will cause large errors. Reference [5] proposed using the finite element method to calculate the magnetic field and loop parameters of the motor, based on which the electric quantity of the internal fault of the stator winding is analyzed. The calculated value is in good agreement with the test. On this basis, this paper proposes a parameterized finite element lattice generation technology to analyze the magnetic field of salient pole motor and calculate the parameters of each stator and rotor circuit. Using this technology, the magnetic field of salient pole motor with any size, pole number and winding form (integer slot and fractional slot) can be analyzed conveniently, which greatly reduces the workload of finite element preprocessing. It is applied to the calculation of loop parameters in the multi loop analysis of stator winding internal faults of large hydro generators such as sf125|96/1560, and the calculation results are satisfactory

2 parameterized finite element lattice generation

a very important step in the finite element calculation of magnetic field is to generate finite element lattice, which includes a large number of node coordinates, element numbers, medium distribution and boundary information. For large hydro generators with complex medium, variable structural dimensions and various winding connections, if the automatic generation technology of finite element lattice is not used, the pre-processing workload will be amazing, In particular, when a certain size of the motor changes, the whole lattice will change. If the structure of the motor changes, such as the number of damping strips, the topological structure of the lattice will also change accordingly. Therefore, this paper proposes a parameterized finite element lattice automatic generation technology to quickly and conveniently analyze the magnetic field of salient pole motors with different size structures, pole pairs and winding connections and calculate their circuit parameters

first, the topological structure of the field is described by polygons, that is, the field is divided into several polygons according to the different media in the field and the special needs of data post-processing. For example, motor rotor yoke, rotor pole, damping strip, excitation winding, stator yoke, stator teeth, upper and lower layers of stator slots, air gap, etc. due to different permeability or different current sources, polygons must be used to distinguish them with clamping phenomenon. If the local structure of the motor somewhere changes, such as the number of stator slots or the number of damping strips increases or decreases, and the number of polygons and their adjacent relationships need to change, a program can be used to pre segment the field

generally, the edge of the polygon describing the magnetic field calculation field of the motor only needs to be composed of straight lines and arcs. For example, the inner and outer diameters of the stator and rotor, the rotor pole arc and the air gap circumference can be represented by arcs, and the arc must be given its center, radius, starting point and ending point. For a polygon, there are concave and convex arcs, which should be considered on each side of the segmented polygon. The number of edge divisions can be determined according to the approximate distribution of the field and the total number of controlled nodes. After all edges are subdivided, the polygons are triangulated one by one according to a certain algorithm

secondly, the change of motor size generally will not change the geometric topological relationship of the polygon, but will change the shape of the polygon, that is, the coordinates of each vertex of the polygon will change. In order to make the software adapt to different motors, the coordinates of the vertex of the polygon must be calculated by the formula, not determined by a fixed value. In this way, the size of the motor can be changed arbitrarily, and the premise is to maintain the correct geometric and topological relationship of the polygon. For this, the system developed in this paper can judge and prompt

the description of boundary is realized through the definition of edges. A kind of boundary and periodic boundary are the collection of several edges respectively. A kind of boundary points and periodic boundary points are generated during the subdivision of polygon edges, and duplicate points need to be eliminated. The set of boundaries may vary with the topology of the field

in the calculation of motor magnetic field, the dissection of air gap has its particularity. Generally, after dividing the stator and rotor, record the information on both sides of the air gap, and then divide the air gap according to this and connect the whole field as a whole. Because the air gap is very small and the field calculation accuracy is required to be high, in addition to the denser lattice, the air gap should be divided into several layers to improve the calculation accuracy

based on the parameterized finite element lattice automatic generation technology, this paper develops a software for calculating the loop parameters of large salient pole motors, and uses it to calculate several large hydro generators. The software has stable performance. Because the parameterized lattice automatic generation technology is adopted, the software is suitable for almost all salient pole motors. In the calculation, only the structural dimensions and relevant electrical parameters of the motor need to be input, and the whole calculation process does not need any intervention

practice shows that the automatic generation technology of parametric finite element lattice introduced in this paper is also applicable to the front of other field calculations

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